The present invention relates to a gas analyzer for continuously determining the concentration of a gas in a gas mixture, the analyzer consisting of an energy source, a measuring cell, a radiation source, a detector and signal-processing devices. The present invention relates furthermore to a measuring cell for the photometric measurement of a gas in a mixture of gases.
The analysis of gases by measuring devices operating on the principle of non-dispersive infra-red spectroscopy (NDIR) has long been known. It has a wide range of applications, including flue gas analysis, process metrology in the field of chemical process engineering, and also recently to an increasing extent the measuring and regulating of air and air quality in buildings.
The fundamental design for a gas analyzer is substantially the same in all cases. The radiation emitted by a radiation source passes through a measuring cell containing the gas to be measured and impinges on a detector. As the radiation travels through the cell, its initial intensity, as emitted by the source, is attenuated by absorption processes. The relationship between the gas concentration to be determined and the attenuation of the intensity is governed by the Lambert-Beer law. The generation of a detector signal with an adequate signal/noise ratio requires modulation of the radiation emitted by the radiation source. The gas to be measured enters the measuring cell either by diffusion or with the help of a pump.
Measuring devices of the above-mentioned type are known from U.S. Pat. No. 5,163,332 and GB Patent 1 398 97. The U.S. Pat. No. 5,163,332 describes an NDIR single beam gas analyzer having a measuring cell which can be operated in the diffusion mode. The cell consists of a closed tube having several discrete gas access openings distributed over the length of the tube. The gas exchange takes place via a membrane which covers the gas openings. The radiation source and the detector are mounted at both ends of the tubular measuring cell. The membrane system makes for a relatively complicated design of the measuring device. GB Patent 1 398 977 also describes a single-beam infra-red photometer for measuring gases, in which the lamp used as a radiation source is supplied with timing pulses by means of an oscillator. The radiation, which is thus modulated with a clock frequency of several Hz, travels along the gas-measuring path and passes through an optical filter, which is transparent to a certain wavelength, before arriving at a radiation-sensitive detector. The measuring cell consists of a tube, closed all round, having a reflecting inner surface. The radiation source and the detector are positioned at the respective ends of the tube. Gas is permitted to enter through a small opening near the optical filter or near the detector. The advantage of using a pulsed light source of this kind is that it permits small, lightweight, cheap, in principle battery-operated and portable but nevertheless efficient gas analyzers to be designed and built.
It is, however, disadvantageous that the known cells can only be used for a narrow measuring range, because the relationship between the concentration of the gas to be measured and the output signals is not linear and the measurement becomes inaccurate with increasing concentration.